Axial flow multi-stage compressors



June 27, 1961 H. R. M. CRAIG 2,990,106

AXIAL FLOW MULTI-STAGE COMPRESSORS Filed Dot. 3, 1957 4 Sheets-Sheet 1 FIG.1

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June 27, 1961 Filed Oct. 3, 195'? H. R. M. CRAIG AXIAL FLOW MULTI-STAGE COMPRESSORS 4 Sheets-Sheet 2 June 27, 1961 H. R. M. CRAIG AXIAL FLOW MULTI-STAGE COMPRESSORS 4 Sheets-Sheet 3 Filed Oct. '5, 1957 mOkOm QOFE. w

June 27, 1961 H. R. M. CRAIG 2,990,106

AXIAL FLOW MULTI-STAGE COMPRESSORS Filed Oct. 5, 195'? 4 Sheets-Sheet 4 g 9 LL H E I ROTATION ROTOR FIG.6

' INVENTOR: HUG-H ROBERT MORTON CRAIG Attorneys United rates Patent 9 The invention relates to axial flow multi-stage com pressors, for example for gas turbine plants and is con- 2,990,106 Patented June 27, 1961 ice whole in the low mass flow range (without necessitating any blow-off).

In particular the surge line of the compressor as a whole which usually follows the stall in one of the lower intermediate stages is shifted in the low mass flow range to the surge line of a higher intermediate stage which is less prone to stalling than the lower stage in this range.

cerned with the problem of reducing the tendency of such compressors to surge in certain operational conditions.

Particularly at low mass flows such compressors are prone to surge, and hitherto this surging has been prevented usually by the use of blow-oif, which although not necessarily causing appreciable losses, complicates the control of the compressor or of the machine, such as a gas turbine engine, co-operating therewith.

According to the present invention, a marked improvement of the surging characteristic of multi-stage compressors has been achieved by providing an axial flow multi-stage compressor comprising in combination: a stator, a rotor, a plurality of rows of stator blades and rotor blades attached to the said stator and rotor alternately in the axial direction thereof, each row of the said stator blades and the successive row of rotor blades forming together a stage of the said multi-stage compressor, consecutive stages of the said multi-stage compressor being matched in radial blade length, and the length of the blades of at least one intermediate stage being shorter than the length of these blades as determined by the said matching of stages. The term matching has the meaning in the art: of designing the consecutive stages of a compressor in such a manner that their aerodynamic loading or contribution to the overal compression ratio of a multi-stage compressor follows a steady law, for example being equal.

For example, the length of the blades of one intermediate stage is made shorter than that of the next higher stage, in contrast with the usual practice of gradually reducing the length of the blades with increasing pressure.

This shortening of the length of the blades of the said middle stage may be efiected by providing the generatrix of either the stator or of the rotor or of both the stator and rotor with a trapexium shaped projection, inward and outward, respectively. This shortening of the length of the blades of a middle stage may be combined with increasing the angle of stagger 'y of the row of blades of said middle stage above that of the preceding and subsequent stages, the angle of stagger 7 being the angle between the chord of the blade and the perpendicular to the cascade of blades of said stage.

Alternatively the angle of incidence i of the rotor blades of said middle stage may be given a negative value of a higher absolute magnitude than that interpolated between the value of the immediately preceding and immediately following stage. The angle of incidence i is the difference between the air angle a and the blade angle 5 at the leading edge of the blade, and is accordingly counted negative if the said blade angle exceeds the said air angle. The air angle a is in turn defined as the angle included between the direction of the air flow relative to the blade and the perpendicular to the cascade, and the blade angle is defined as the angle between the tangent to the mean camber line at the leading edge and the said perpendicular to the cascade. The profiles of the stator and rotor blades, respectively, may be left substantially identical in these consecutive stages.

The result of the measures of the invention is a marked extension of the surge-free range of the compressor as a In order that the invention may be clearly understood, and readily carried into effect, an embodiment and modifications thereof will be described hereinafter by way of example with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic longitudinal section through the blading of the intermediate stages of an axial flow multi-stage compressor according to the invention,

FIG. 2 is a graph plotting total head pressure ratio over mass flow,

FIG. 5 shows diagrammtically a part development of g the stator blading and rotor blading of three consecutive stages,

FIG. 6 shows diagrammatically a part development of a rotor blading and of the stator blading of the subsequent stage, the said rotor blading having a negative angle of incidence.

In FIG. 1 the lowest and highest stages of a multi-stage axial flow compressor having the axis of rotation 00 are omitted, and only the blading of some intermediate stages is shown, 1, 3, 5, 7, 9 being rows of stator blades, and 2, 4, 6, 8, 10 rows of rotor blades of these stages. The rotor R is assumed to have a cylindrical circumference, and the stator S has a generatrix, which with conventional matching as defined hereinabove of stages is indicated in dotted lines for the blading 3, 5 and 7, and for the shortened blading of these stages according to the invention is indicated in full lines to have a substantially trapezium-shaped inward projection.

It will be noted that the blades 5, 6 are of a shorter radial length than the blades 7, 8 of the next higher stage of the compressor.

The abrupt change of blade lengths as shown in FIG. 1 facilitates development work on existing compressors with a View to improving its overall surge characteristic, so that the few intermediate stages concerned only to be modified. This waisted compressor has moreover been found superior to any gradual change in blade lengths over the whole middle stages of the compressor, wherein the blade lengths, although reduced as compared with those arrived at by conventional matching as defined hereinabove are yet diminishing from one stage to the next higher one.

Alternatively or additionally, the stagger of the blades of one of the intermediate stages may be different from the stagger interpolated between that of the immediately adjacent stages. For example, the rows of blades of several successive stages may have a substantially identical form, the rows of a middle stage thereof being set on a higher stagger than those of preceding and subsequent stages. In FIG. 5 it is shown that the intermediate stage composed of the stator blades 5 and rotor blades 6 has a higher stagger than the two adjacent stages composed of the stator blades 3 and rotor blades 4, and the stator blades 7 and rotor blades 8, respectively.

The rotor blading 6 may have a negative angle of incidence of a higher absolute value than that interpolated between that of the immediately preceding and immediately following stage.

A part development of a rotor blading 6 having a negative angle of incidence i is illustrated in FIG. 6 in conjunction with the part development of the subsequent stator blading 7, the notation of FIG. 6 corresponding to that of FIG. as described hereinabove.

Referring now to FIG. 2, a, a, a", a' are graphs of total head pressure ratio plotted over mass-flow for,c ertain rotational speeds, and b is the surge line ofthe mulfi stage compressor as a whole with blading of a length corresponding to the dotted line in FIG. 1; The intersectingpoints of b with a, a,- a", a' show where/surging occurs at various rotational speeds.

c and d are lines indicating the stall points oftwo consecutive stages of the intermediate range as indicated in full lines in FIG. 1 and e is the improved surge line of the compressor, which proves to be near the stall line c for the lower intermediate stage in the upper ranges of mass flow, pressure ratio and rotational speed and nearer the stall line d for the next higher intermediate stage in the lower ranges of mass flow,,pressure ratio and rotational speed. The speed lines a, a, a, a are changed to f, f, f", f' by the reduction of blade height. The marked improvement in surge line is. evident, the surge-free range below and to the right of the surge line 2 being substantially extended compared to surge line b.

FIG. 3 shows a modification in which the stator S has a straight line generatrix, while the rotor R has a generatrix with a substantially trapezium-shaped outward projection.

FIG. 4 shows another modification in which the generatrices of both the stator-S and the rotor R have a substantially trapezium-shaped inward and outward projection, respectively.

It will be noted that also in these modifications the blades 5, 6 are of shorter radial lengththan the blades of the next higher stage.

The eificiency of the compressor is not appreciably affected by the modification of the intermediate stages as described.

What I claim as my invention and desire to secure by Letters Patent is.

1. An axial flow multi-stage compressor comprising in combination: a stator, a rotor, a plurality of rows of stator blades and rotor blades attached to the said stator and rotor alternately in the axial direction thereof, each row of the said stator blades and the successive row of rotor blades forming together a stage of the said multi stage compressor, the rotor blades of consecutive stages of the said multi-stage compressor being matched in angle of incidence so as to operate on corresponding points of their respective lift graph plotted over angles of incidence, and the rotor blades of at least one middle stage having a negative angle of incidence of a higher absolute value than that interpolated between that of the immediately preceding and immediately following stage.

2. An axial flow multi-stage compressor comprising in combination: a stator, a rotor, a plurality of rowsof sta-. tor blades and rotor blades attached to the said stator and rotor alternately in the axial direction thereof, each row of the said stator blades and the successive row of rotor blades forming together a stage of the said multistage compressor, the rotor blades and stator blades, respectively, of consecutive stages of the said multi-stage compressorbeing matched in blade stagger so as to operate at equivalent aerodynamic loading, and the rotor blades and stator blades, respectively, of at least one middle stagehaving a blade stagger higher than the blade stagger interpolated between those of the immediately preceding and immediately following stage.

3. An axial flow multi-stage compressor comprising in combination: a stator, a rotor, a plurality of rows of stator blades and rotor blades attached to the said stator and rotor alternately in the axial direction thereof, each row of the said stator blades and the successive row of rotor blades forming together a stage of the said multistage compressor, the rows of blades of several successive stages having a substantially identical profile and the row of rotor blades and stator blades, respectively, of a middle stage thereof being set at a higher stagger than those of the preceding and subsequent stages.

4. An axial flow multi-stage compressor comprising in combination: a stator, a rotor, a plurality of rows of stator blades and rotor blades attached to the said stator androtor alternately in the axial direction thereof, each row of the said stator blades and the successive roW of rotor blades forming together a stage of the said multistage compressor, consecutive stages of the said multi stage compressor being matched in radial blade length so as to operate at equivalent arerodynamic loading, and the length of the blades of at least one middle stage being made shorter than the length of these blades as determined by the said matching of stages.

5. An axial flow multi-stage compressor as claimed in claim 4, having the length of the blades of the said middle stage shorter than the length of the next higher stage.

6. An axial flow multi-stage compressor as claimed in claim 5, wherein the generatrix of the said stator has a substantially trapezium-shaped inward projection in the range of the said middle stage having shorter length of blades than the next higher stage.

7. An axial flow multi-stage compressor as claimed in claim 5, wherein the generatrix of the said rotor has a substantially trapezium-shaped outward projection in the range of the said middle stage having shorter length of blades than the next higher stage.

References Cited in the file of this patent UNITED STATES PATENTS 2,830,754. Stalker Apr. 15, 1958 2,846,136 Zaba Aug. 5, 1958 FOREIGN PATENTS 1,060,376 France Nov. 18, 1953 

